In the color diffusion transfer photography, it is well known to use a redox compound releasing a diffusible dye as a dye image-forming compound (a coloring material). Such redox compound includes so-called negative and positive type coloring materials. When a positive transfer image is obtained with the negative type coloring material, a positive emulsion is used or other reversing mechanism is needed. On the other hand, when a positive transfer image is obtained with the positive type coloring material, a negative emulsion can be used. Thus, the positive type coloring material is variously advantageous, for example, in that it can provide a light-sensitivity material having high sensitivity.
As such positive type coloring materials, there can be exemplified immobile compounds as disclosed in Patent Application (OPI) Nos. 111628/74 and 4819/77 to Hinshaw and 63618/76 to Fields (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). These immobile compounds can release diffusible dyes upon an intermolecular nucleophilic reaction in the reduced state in the presence of an alkali. On the other hand, when they are oxidized in the light-sensitive material by a redox reaction, the rate of releasing the dyes decreases. Such a property of these immobile compounds can be utilized to imagewise form a positive transfer image. However, these immobile compounds leave much to be desired. For example, since oxidation and alkaline hydrolysis compete with each other in these immobile compounds, a possible mistiming of the both reactions or the like causes fog and deteriorates discrimination. Furthermore, since the released dyes have no water-soluble groups, they are poor in diffusability, and even once diffused into a mordant layer, the released dyes (image-forming dyes) are hard to fix (i.e., easy to leave) before mordanting.
As the positive type coloring material which solves the above disadvantages, a "BEND compound" is disclosed in U.S. Pat. No. 4,139,379.
This compound can eliminate the above disadvantages, including the deterioration of discrimination, because its nucleophilic groups become precursors requiring the acceptance of one electron before an intermolecular nucleophilic displacement.
Specifically, the BEND compound becomes useful only when it is combined with an electron donor (reducing agent) which can reduce it into a state such that the intermolecular nucleophilic displacement takes place. Accordingly, the BEND compound is disadvantageous in that the release of the dyes is delayed in proportion to its reaction with the reducing agent.